The past decade has witnessed dramatic changes in the oil and gas industry with the advent of deep-water exploration and production. Deep water exploration and production favours the formation of solid ice-like materials known as gas hydrates or clathrates. Clathrates are formed when polar molecules such as Water (H2O) align through hydrogen bonding effects under the conditions of high pressure and (often low) temperatures typical of such deepwater locations to form hollow cage like structures that can trap and hold Carbon Dioxide (CO2), Methane (CH4) or other gases. These solid materials remain until they are subject to a change in their formation conditions (e.g. lower pressure or a higher temperature) that causes them to dissociate and release the trapped gases back to the surrounding atmosphere. There are several types of hydrate structure e.g. sl and sll that can form depending on the conditions of temperature, pressure and hydrocarbons present. It is known that sl hydrates form with lower molecular weight hydrocarbons such as Methane (CH4), and that sll hydrates form preferentially with the presence of heavier hydrocarbons such as Propane (C3H8) or n-Butane/Iso-Butane (C4H10) making deepwater exploration where these are prevalent in the production streams potentially even more hazardous.
One of the major challenges within deepwater field development is to ensure unimpeded flow of hydrocarbons to the host platform or processing facilities; the early detection of the formation and managing the remediation of solids such as hydrate, wax, asphaltene and scale is key to the viability of developing deepwater prospects.
One of the problems other than blockage is the movement of the hydrate plugs in the pipeline at high velocity, which can cause rupture in the pipeline. Any blockage in an oil/gas pipeline due to hydrate is a serious threat to capital equipment and personnel safety. A number of strategies exist to inhibit or stop hydrate formation within transfer line or process facilities and one traditional approach is to remove or change one of the elements that favours hydrate formation such as temperature or pressure.
Examples of such strategies include thermal insulation or the external heating of transfer lines, water removal from natural gas using glycol dehydration systems, lowering operating pressure (mainly for removing blockage) or chemical approaches such as adding inhibitor materials to the system. Although often effective in reducing the formation of solids or treating the problem after the event, they increase OPEX or CAPEX. Despite the above prevention techniques, hydrates could form due to changes in the system conditions, inhibitor injection pump malfunction, error in calculating the amount of inhibitor required, etc. Currently there is no reliable technique in predicting the early formation of the solid hydrates themselves.
In support of these strategies, attempts have been made to detect early hydrate formation and a conference paper published by Tohidi et al. (SPE94340, EAGE Conference, Madrid, Spain, June 2005) describes a method for the early detection of hydrates based on measuring the dielectrical properties of reservoir fluids. The technique proposed by Tohidi et al. detects hydrate history by measuring the dielectric constant (permittivity) of aqueous samples. This method shows a high sensitivity to both chemical and physical contaminations of the sampling fluids that includes the presence of “micro bubbles” and other chemical additives; this can lead to false positive results that may affect its feasibility and reliability, which possibly hinders it for online application.
An alternative approach to measuring electrical properties was proposed by Tohidi et al. and is revealed in patent application, WO2006/054076. The method is based on identification of water memory by freezing point measurements. One drawback of this method is that freezing point measurements have a highly stochastic nature requiring a certain number of measurements to achieve the desired reliability/probability for hydrate early warning. Moreover, this freezing-point-based hydrate memory could be easily weakened even fully destroyed by the presence of certain hydrate inhibition additives.
There is no method or apparatus available for detecting either the onset or the early formation of hydrates in practice that could be used to inform the existing hydrate reduction strategies briefly outlined above—such a system would potentially reduce the need for high CAPEX heating/insulation, minimise the energy input to the heating systems, reduce inhibitor chemical usage and increase the safety of the personnel and capital equipment in deepwater exploration & production.